The present invention relates to a method of producing a recombinant protein in a host cell comprising adding Polyethyleneimine (PEI) during cell culture. Addition of PEI to the cell culture as a fermentation enhancer can result in reducing the viscosity of the cell culture, and/or increasing the extracellular concentration of the recombinant protein, and/or reducing the duration of cell culture to the point of harvest or protein recovery.